Particles fall through collision object
To avoid particles to intersect with the ground, check Size Deflect checkbox in particle system Physics settings. This is not a perfect solution either, cause the particles will hover over the ground plane a little, so what you can do is to use this plane as collision box, which won't be rendered and it's duplicate without collision added, which you move closer/furhter away to the particles and which will be rendered. If you need even more physically accurate interactions between leaves and a plane, you would need to use separate objects (which can be created from particles by Shift+CTRL+A) and make them rigid bodies.